The XtAddGrab
function appends the widget (and associated parameters) to the modal
cascade and checks that exclusive is True if spring_loaded is True.
If these are not True, XtAddGrab generates an error.

The modal cascade
is used by XtDispatchEvent when it tries to dispatch a user event. When
at least one modal widget is in the widget cascade, XtDispatchEvent
first determines if the event should be delivered. It starts at the most
recent cascade entry and follows the cascade up to and including the most
recent cascade entry added with the exclusive parameter True.

This subset
of the modal cascade along with all descendants of these widgets comprise
the active subset. User events that occur outside the widgets in this subset
are ignored or remapped. Modal menus with submenus generally add a submenu
widget to the cascade with exclusive False. Modal dialog boxes that need
to restrict user input to the most deeply nested dialog box add a subdialog
widget to the cascade with exclusive True. User events that occur within
the active subset are delivered to the appropriate widget, which is usually
a child or further descendant of the modal widget.

Regardless of where on
the screen they occur, remap events are always delivered to the most recent
widget in the active subset of the cascade that has spring_loaded True,
if any such widget exists.

The XtRemoveGrab function removes widgets
from the modal cascade starting at the most recent widget up to and including
the specified widget. It issues an error if the specified widget is not
on the modal cascade.